Ionization-type smoke detectors and photoelectric-type smoke detectors are currently available. In an ionization-type smoke detector, a very low ionic current is generated in the detector's detection chamber and the current flows from one side of the detection chamber to the opposite side thereof. A stream of air also flows through the detection chamber. When particles, including smoke particles, are entrained in the stream of air, these particles alter the flow of the ionic current. Then, when a change in the ionic current flow is detected by a sensor that is included in the smoke detector, the sensor activates an alarm indicating the presence of smoke particles.
In a photoelectric-type smoke detector, a light source, typically in the form of a Light Emitting Diode (LED), and a light sensor are mounted at an acute angle relative to each other inside of the detector's detection chamber. As such, the light sensor is shielded from stray light from the light source. When smoke particles enter the detection chamber, light emitted by the light source is scattered by the smoke particles, the scattered light is detected by the light sensor and an alarm is activated.
Ionization-type smoke detectors are sensitive to relatively small (i.e., less than about 1.0 micron in diameter) airborne particles produced during the early phases of flaming fires. As such, ionization-type smoke detectors typically respond to flaming fires faster than do photoelectric-type smoke detectors. However, some types of smoke particles (i.e., smoke particles that do not disrupt the ionic current very much) are more likely to be sensed by a photoelectric-type smoke detector than an ionization-type smoke detector.
In view of the above, when an ionization-type smoke detector is configured to be sensitive even to smoke particles that only slightly disrupt the ionic current therein, the detector will be overly sensitive to the presence of smoke particles that substantially disrupt the ionic current. Thus, ionization-type smoke detectors tend to have a high incidence of false alarms. For example, ionization-type smoke detectors sound alarms when they detect small, non-smoke particles such as cooking, cleaning fluid and paint fume particles.
Photoelectric-type smoke detectors, on the other hand, respond relatively quickly to relatively large (i.e., greater than about 1.0 micron in diameter) smoke particles generated by smoldering fires. However, because the color of the smoke particles greatly affects the amount of light that the particles scatter, photoelectric-type smoke detectors respond to the presence of black smoke much more slowly than they respond to the presence of white smoke.
In addition to the shortcomings mentioned above, ionization-type and photoelectric-type smoke detectors also suffer from a number of other shortcomings. For example, both of these types of detectors are highly sensitive to dust and dirt accumulation in their detection chambers.
In ionization-type smoke detectors, the presence of dust particles decreases conductivity and thereby distorts the ionic current flow. In photoelectric-type smoke detectors, dust particles that accumulate on the detection chamber walls scatter light onto the light sensor and thereby cause false alarms and increase background noise. Further, when a dust particle layer accumulates on the sides, top and/or bottom of the detection chamber in a photoelectric-type smoke detector, the presence of the layer increases the reflectivity of the wall relative to a conventional black detection chamber wall. Hence, stray light propagating from the light source reflects off of the dust layer and increases the amount of light that reaches the light sensor. The light sensor, in turn, responds to this increase by producing an output that indicates the presence of smoke particles and consequently activates an alarm.
Because the presence of dust in smoke detectors cannot be avoided, most commercial fire codes mandate that regular testing and cleaning procedures be instituted to avoid excessive dust accumulation. Unfortunately, cleaning a detector is expensive, inconvenient and/or time-consuming. Therefore, some smoke detectors have been designed to minimize the amount of dust that settles on the walls of the detection chamber of a smoke detector. However, the overall cost and complexity of such smoke detectors is relatively high.
Among the other shortcomings of ionization-type and photoelectric-type smoke detectors are their sensitivities to wind and outside light sources. In view of these shortcomings, ionization-type detectors cannot be used in air ducts or near wind drafts because the excessive air flow can blow the ions out of the detection chamber. To reduce the effect of wind drafts and outside light, photoelectric-type detectors generally include partitions and walls that block dust and light emitted by outside light sources. However, these partitions and walls often significantly decrease the flow of air carrying smoke particles into the detection chamber, thereby reducing the responsiveness of the detector.
One attempt to provide a smoke detector with an increased sensitivity and a reduced incidence of false alarms entailed creating a combination ionization-type/photoelectric-type smoke detector. When combined in a logical “OR” configuration, the combination smoke detector responded more rapidly to many different types of smoke. However, the incidence of false alarms increased. When combined in a logical “AND” configuration, the incidence of false alarms was reduced. However, the combination smoke detector displayed decreased sensitivity to many of the different types of smoke. Therefore, neither combination was entirely successful.
What is needed, therefore, is an improved smoke detector that is consistently sensitive to a wide range of smoke types (e.g., small-diameter smoke particles, large-diameter smoke particles, smoke particles of different colors) while exhibiting a reduced incidence of false alarms. What is also needed are methods for detecting this wide range of smoke types while also reducing the incidence of false alarms.